This invention relates to RF coil structure for intra-cavity use in magnetic resonance imaging (MRI).
Magnetic resonance imaging is used in medicine to produce images of the internal organs of a patient being examined. In MRI, a static magnetic field is applied to the body of the patient to define an equilibrium axis of magnetic alignment in the region of the body being examined. A radio frequency field is then applied to the region being examined in a direction orthogonal to the static magnetic field direction to excite magnetic resonance in the region. This resonance produces signals in RF coils placed adjacent the body. Normally separate coils are used for excitation and detection, although the same coil or coils may be used for both purposes. The detected signals are processed to produce signals representing an image of the patient's body and this image is visually displayed. In so-called interventional MRI, RF coils are introduced into the body, for example, inside a catheter, which may be passed along the male or female urethra to examine the bladder, or may even be passed down a blood vessel.
There are been a recent growth of interest in the use of MRI RF coil probes within the body so as to exploit the higher local signal to noise ratio (SNR) which may be achieved when compared to external coils.
Such insertable coils may be solid in structure (deSouza N. M., Hawley, I. C., Schwieso J. E., Gilderdale D. J., Soutter W. P., AJR 163: 607-612 (1994); deSouza N. M., Gilderdale D. J., Puni R., et al, JMRI 6: 801-804 (1996); and deSouza N. M., Puni R., Kmoit W. A., et al, J. Comput. Assist Tomogr. 19: 745-751 (1995)), or may be made flexible, as may be advantageous for vascular microscopy (WO-A-96/38083, Atalar E., Bottomley P. A., Ocali O., et al, MRM 36: 596-605 (1996)).
The elongated loop RF coil described in the last mentioned reference is shown in FIG. 1, and consists of a pair of conductors 1, 2 joined by a web 3. The structure has a higher flexibility in the z-y plane than in the z-x plane. Depending on the length of the coil and the radii and direction of the bends to be negotiated, this lack of flexibility may prove to be unacceptable. Flexibility would be improved by the use of a twisted pair but, for adjacent loops, the current induced by the protons during MRI relaxation is in opposite senses (unless the area being imaged is so small that it only lies adjacent to a single loop), so that such a twisted pair would not be able to image anything at a distance from it because of cancellation, and would only be able to image the path of a catheter e.g. down a blood vessel.